One of the most ubiquitous circuits used in the field of integrated circuit is the so-called “bandgap” voltage reference, which strives to produce a temperature independent voltage. Such bandgap voltages are used, for example, to produce reference voltages and bias currents for a wide variety of analog circuits, and can be found in wide variety of circuits including memory circuits, data converter circuits, voltage regulators, power supplies, and RF circuits.
The temperature independent voltage of the bandgap voltage reference is generally produced by combining an output of a circuit that generates a DC signal that is proportional to absolute temperature (PTAT) with an output of a circuit that generates a DC signal that is complimentary to absolute temperature (CTAT). A common method of producing a PTAT DC voltage is to generate a voltage difference between the base-emitter junctions of two bipolar transistors operating a different collector current densities, while a common method of producing a CTAT signal involves monitoring a base-emitter voltage of a bipolar transistor or a junction voltage of a diode, which are generally inversely proportional to temperature.
While the general concept behind producing a temperature independent voltage is straightforward, the practical implementation of such circuits is challenging. When implemented using semiconductor processes that exhibit statistical variation in terms of process parameters and feature geometry, voltage reference circuits become prone to part-to-part and lot-to-lot variation that affects both in the nominally generated DC reference and the performance of the voltage reference over temperature.